With the development of radio and space technologies, several satellites based navigation systems have already been built and more will be in use in the near future. One example of such satellites based navigation systems is Global Positioning System (GPS), which is built and operated by the United States Department of Defense. The system uses twenty-four or more satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. These satellites are placed in six different orbits such that at any time a minimum of six satellites are visible at any location on the surface of the earth except in the polar region. Each satellite transmits a time and position signal referenced to an atomic clock. A typical GPS receiver locks onto this signal and extracts the data contained in it. Using signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time. The Russian built GLONASS and the European Union proposed Galileo are the two other important satellite based navigation systems.
A GPS receiver has to acquire and track at least four satellite signals before starting to compute the position which is also known as position fix. The time required to fix the position depends upon how fast the satellite signals can be acquired and tracked. This signal acquisition involves a search of the carrier frequency including the Doppler due to the motion between the satellite and the navigation receiver. In addition to this frequency search the receiver should also search for the actual code phase of the received signal. Thus the search is a two dimensional search and takes most of the Time-To-First-Fix (TTFF). It is always desirable to lower this TTFF so that a fast position estimation is possible. This resulted in the operation of the receiver in different modes such as ‘hot start’, ‘warm start’ and ‘cold start’. In the ‘hot start’ mode the receiver has current ephemeris and the position and time, and therefore the list of the visible satellites and the Doppler associated with each satellite may be determined. Thus only the visible satellites are searched. Since the carrier frequency including the Doppler is known, the frequency search range or the number of frequency bins that need to be searched is also decreased. Thus the ‘hot start’ results in a shorter TTFF of usually around 7-10 seconds. In ‘warm start’ the receiver has non-current ephemeris but the initial position and time are known as accurately as in the case of ‘hot start’. Thus the Doppler cannot be determined accurately as in the case of ‘hot start’ due to the higher number of frequency bins that need to be searched. Thus the ‘warm start’ TTFF has a typical value of 30-35 seconds. In the third mode, known as ‘cold start’, the receiver has no knowledge of position, time or ephemeris but may have almanac in the memory. Thus a search for all the satellites in the constellation is required. In addition to this the Doppler frequency is also not known. As a result the frequency search range in this case is far wider with a large number of frequency bins. Thus the receiver in the ‘cold start’ mode has the highest TTFF, which can be as large as 80-100 seconds.
It is not always possible to store the above information in the receiver at all times or the receiver might have moved over a long distance before the receiver is powered on. Under such conditions it is necessary to resort to ‘cold start’.
Therefore, there is a need to reduce the ‘cold start’ TTFF of navigation receivers.